1. Field of the Invention
This invention relates to bonding of input/output electrical connection pins and other components to a chip carrying substrate in an electronic system. More particularly, this invention relates to electronic circuit interconnection bonding techniques for fabrication of a chip carrying substrate and its pins in a manner compatible with the continuous reheating of the chips.
An object of this invention is to be able to attach or replace a chip on the substrate without disturbing the bond between pins connected to the substrate or the surfaces to which the substrate itself is bonded. In other words, the object is to bond pins, etc., to the substrate and the substrate to its carrier with a bonding material having a melting point such that the structure is unaffected by melting of the solder connections retaining the chips upon the substrate.
Brazing of elements to electronic chip carrying substrates such as multilayered ceramic substrates requires providing a brazing or soldering material which remains strong at high temperatures used for rework, i.e., removal and replacement of chips on the substrate, with the heat being required for heating the lead-tin solder balls supporting the chips. A standard solution to this problem is to use gold-tin solder whose melting point subsequent to brazing is higher than the initial 280.degree. C. melting point.
Here because circuit connection pins have tended to tilt out of alignment during rework heating, and because problems have arisen in flange sealing with numerous cycles of rework, the braze alloy is modified during brazing by adding a Group IB metal, i.e., gold, copper and/or silver to the brazing material (which raises the melting temperature subsequent to bonding by increasing the amount of the higher melting point .beta. phase of the alloy in the braze material) and a metal such as nickel, palladium or another Group VIII metal (which will tend to draw tin out of the melt or to getter the tin thereby promoting the apparent ratio of gold or IB metals to tin and the formation of the .beta. phase of the Au-Sn alloy thereby tending to raise the melting temperature of the brazed joint subsequent to cooling, even further as well as thickening the braze material). In one aspect of the method, a gold preform is plated with nickel and placed in juxtaposition with the surface to be brazed along with the brazing material in order to add nickel and gold to the melt of the brazing material almost simultaneously.
2. Background Art
U.S. Pat. No. 3,648,357 of Green, "Method for Sealing Microelectronic Device Packages" is concerned with hermetically sealing of a Kovar and glass housing and a Kovar alloy (Ni, Fe, Co) cover together with an eutectic gold-tin solder. In the past Green stated that (Col. 1, lines 12 et seq.) such packages had been sealed by placing ". . . a solder preform between the periphery of the edges of the housing and the cover to be sealed together." Then the parts to be soldered together were "placed in a sealing machine and . . . heated to a sufficient temperature to cause the solder to fuse onto the cover and housing. Unfortunately, imperfect hermetic seals result in a substantial portion of the packages so sealed. Not only may leaks in the seal result due to voids therein, but the temperature necessary to melt the solder and form the seal may be sufficient to damage the microelectronic device contained in the package . . . ". In Green, the approach was to coat an 80:20 Au:Sn preform upon both the housing flange and upon the mating surface of the cover which fits upon the housing. In that way, the preform solder halves can be joined together at a low temperature at which the solder melts requiring little heat so as to reduce damage to the microelectronic device. However, since the Kovar alloy reacts badly with Au:Sn solder, the surfaces of Kovar alloy are first coated with gold by plating. Then the preforms are soldered to the plated housing and cover. The pretinning had the disadvantage that it raised the temperature of melting of the solder to the degree that the gold is melted from the Kovar plated surfaces. The purpose of the Green patent was to keep the melting point down to about 330.degree. C. rather than the 400.degree. C. temperature which would result with melting of the gold into the solder under equilibrium conditions. Use of the lower temperature assures that at equilibrium only a portion of the plated gold "will be dissolved into the solder."
A current practice in the industry is to join pins composed of Kovar alloy with a thin film of palladium with nickel pads deposited upon a thin film of molybdenum. The solder used is an Au:Sn brazing alloy. The Pd layer melts away after about four of the ten solder reflows one would expect under current practices in microelectronic circuit manufacture. As a result, a problem of leaching of Ni from the Kovar alloy pins and the pad below into the braze which is unacceptable since it results in poor adhesion, causing rejection of the entire device. Accordingly, it is the desideratum to be able to reheat the braze repeatedly during such reflows with freedom from the remelting of the braze and consequent introduction of impurities into the braze which would ruin the product because of such weakening of the interface with the Kovar alloy.
Hitt et al, "Brazing Alloys for I/O Pins to Multilayer Ceramic Electronic Modules", IBM Technical Disclosure Bulletin, Vol. 21, No. 9, 3950 (Feb. 1979) describes braze alloys of weight percentages of as follows: 75 Au, 16.2 Sn, and 8.8 Pd; and 80 Au, 15 Sn, and 5 Pd used for brazing I/O pins and frames for ceramic modules. No additional metal is supplied for the purpose of altering the melting point of the liquid alloy in the braze subsequent to its liquefaction.
Ainslie et al, "Au/Sn/Ag Braze Alloy", IBM Technical Disclosure Bulletin, Vol. 21, No. 8, 3118 (Jan. 1979) describes a braze alloy for electronic packaging for Be-Cu contact pins or Kovar (Ni, Co, Fe) pins brazed with 67 Au/15 Sn/18 Ag. In either case there is either a Group IB of the Period Table of the Elements metal (Cu) or a Group VIII metal (Ni, Fe, Co) in the Be-Cu or Kovar pins which fails to have the effect provided in accordance with the present invention. In addition, the alloy of the ratio of 67 Au to 15 Sn is not the 80/20 ratio with the eutectic point permitting low temperature melting of the alloy, in the phase diagram at about 280.degree. C. which is enhanced by the added Group IB metal raising the liquidus temperature while the Group VIII metal removes Sn from the alloy. Here lowering the Sn in the alloy tends to move the alloy towards a lower melting point of the alloy from a high of about 400.degree. C. at 67% Au down near 280.degree. C. as the copper and nickel operate to move the alloy down to the eutectic point. Thus, the temperature effect is the opposite of what is desired here. The 18% of Ag in the alloy, however, acts as a substitute for gold since it is a Group IB metal which raises the melting point of the resulting alloy.
Another article of Ainslie et al, "Au/Sn/Ag Braze Alloy", IBM Technical Disclosure Bulletin Vol. 21, No. 8, 3117 (Jan. 1979) describes a braze alloy of 70 Au/25 Sn/5 Ag for pin or electronic component brazing. The alloy can withstand chip joining cycles of reheating at 350.degree. C. The liquidus is at 358.degree. C. The total of Group IB metals is 75 weight percent as compared with the 25 weight percent of Sn. The pins brazed are Be-Cu pins. This alloy has the disadvantage of not having the original low melting point of an 80:20 Au:Sn brazing alloy.
Slayton et al, "Brazing Process for 57.2 Au/30.8 Sn/12.0 Ag Alloys", IBM Technical Disclosure Bulletin Vol. 21, No. 8, 3119 (Jan. 1979) describes a brazing alloy for electronic packaging components for pin brazing. It is indicated that there is Ni which if unreacted remains upon the substrate. It also states that too early a "slow-down in cooling rate results in excessive Ni consumption and poor mechanical properties."
In accordance with this invention, joining of objects together is performed by brazing of eutectic Au:Sn solder at a low temperature, and in the process elevating the melting point substantially above the eutectic point subsequently to avoid pin tilt of brazed pins, braze strength degradation and other relative motion between the parts being brazed together during reflow operations upon the micro-electronic circuit.
The process of this invention involves brazing a first surface to a second surface by means of a gold-tin brazing solder, characterized by providing a source of a Group IB metal and a source of a Group VIII metal in combination with the brazing solder, to come into contact with the first and second surfaces during brazing, whereby the melting temperature of the brazing solder is raised substantially during brazing to a temperature substantially above the initial melting temperature of the solder which is reflowed repeatedly whereby the solder joint formed by the braze is unafffected by reflow cycles. This is accomplished by increasing the amount of .beta. phase (.perspectiveto.Au-10 wt. % Sn) in the braze, which greatly increases the melting point as the percentage of Au-Sn alloy in the braze is decreased.